单例模式三种实现

//饿汉式
public class Singleton {
    private static final Singleton s = new Singleton();
    private Singleton(){}
    public static Singleton getInstance(){
        return s;
    }
}

//懒汉式
class Singleton2{
    private static Singleton2 s = null;
    private Singleton2(){
    }
    public static synchronized Singleton2 getIntence(){
        if(s==null){
            s = new Singleton2();
        }
        return s;
    }
}

//双重加锁检查
class Singleton3 {
    private static volatile Singleton3 s = null;
    private Singleton3() {
    }
    public static Singleton3 getInstance() {
        if (s == null) {//提高效率
            synchronized (Singleton3.class) {
                if (s == null) {
                    s = new Singleton3();
                }
            }
        }
        return s;
    }
}